This application is based upon and claims priority from Japanese patent application No. Hei 11-3575, filed Jan. 11, 1999, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an ignition coil, such as a stick-type ignition coil, which is installed in a plug hole for each cylinder of an engine.
2. Description of Related Art
One type of known stick-type ignition coil is filled with epoxy resin to insulate and fix a primary winding, a secondary winding and a cylindrical core disposed in a cylindrical coil casing.
The epoxy resin to be filled in the coil casing shrinks when it is hardened after filling. Accordingly, a stress caused by the hardening and shrinkage of the epoxy resin is applied to the plastic parts in the coil casing such as a secondary spool, and the stress remains as it is. Furthermore, the stress applied to the plastic parts may be increased when the epoxy resin is cooled and shrinks after the engine stops if the ignition coil is mounted on the engine. Accordingly, a crack on the plastic parts may be caused by the shrinkage of the epoxy resin, and such crack may decrease the insulation performance.
Especially, an outside dimension of a stick-type ignition coil is regulated by an inside dimension of a plug hole. Thus, the plastic spool has a reduced thickness to obtain a winding space for the coil. Accordingly, mechanical strength of the spool is relatively weak, and an allowable stress of the spool against the shrinkage of the epoxy resin is relatively small. Therefore, it is necessary to reduce the shrinkage force of the epoxy resin applied to the spool in order to prevent the crack of the spool.
Further, the shrinkage force of the epoxy resin applied to the secondary spool locating inner than the primary spool tends to be greater than that applied to the primary spool. Accordingly, the crack is likely to be generated on the secondary spool.
The present invention is made in light of the foregoing problem, and it is an object of the present invention to provide an ignition coil which prevents a crack of a part of the ignition coil caused by a shrinkage force of filling resin and which improves the insulation reliability.
According to an ignition coil of the present invention, the ignition coil includes a partition member provided in an upper portion of a coil casing for dividing a resin filler locating above a primary coil and a secondary coil into an inside and an outside. Accordingly, the partition member reduces the shrinkage force between the outside resin filler and the inside resin filler. Thus, stress of plastic parts caused by the shrinkage force is reduced, and a crack of the plastic parts is prevented, and the insulating reliability is improved.
According to another aspect of the present invention, if the partition member is made of made of a material whose adhesive strength against the resin filler is substantially weak such that each of the resin filler and the partition member is capable of expanding and shrinking independently. Accordingly, the resin filler is easily peeled from the partition member when the resin filler shrinks or expands, thereby reduces stress applied to the plastic parts.